METHOD FOR CONTROLLING Ti CONCENTRATION IN STEEL, AND METHOD FOR PRODUCING SILICON-DEOXIDIZED STEEL

ABSTRACT

Disclosed is a method for controlling a Ti concentration in a steel when manufacturing a silicon-deoxidized steel comprising 0.1 to 3% by mass of Si and 0.0001 to 0.005% by mass of Al by ladle refining of a molten steel, the method including the step of:
         adding an oxide including TiO 2  to a slag in a ladle during the ladle refining, wherein the slag produced at end of the ladle refining satisfies formulas (1) to (7) below:       

       0.5≦CaO/SiO 2 ≦1.8  (1)
 
       4% by mass≦Al 2 O 3 ≦20% by mass  (2)
 
       MgO≦15% by mass  (3)
 
       1.5% by mass≦TiO 2 ≦10% by mass  (4)
 
       CaO+SiO 2 +Al 2 O 3 +MgO+TiO 2 ≧90% by mass  (5)
 
       0.4≦TiO 2 /MnO≦5  (6)
 
       1≦TiO 2 /T.Fe≦10  (7)
 
     where a compound represented by a chemical formula represents the content of the compound in percent by mass; and T.Fe represents the total concentration, in mass ratio, of Fe contained in Fe oxides in the slag.

TECHNICAL FIELD

The present invention relates to a method for controlling a Ti concentration in a steel when manufacturing the steel, such as a silicon-deoxidized steel, and a method for producing a silicon-deoxidized steel using the controlling method.

BACKGROUND ART

Conventionally, in ladle refining, a method for deoxidizing a steel is known to add aluminum (Al) or silicon (Si) to a molten steel tapped from a converter or an electric furnace. In such deoxidization treatment of steel, techniques have been developed to minimize inclusions in the steel after deoxidization and to optimize the components of the steel (see Patent Documents 1 to 6, etc.)

For instance, Patent Document 1 discloses a technique that sets a slag composition to satisfy the relationships below: CaO+Al₂O₃≧80%, SiO₂≦3%, CaO/Al₂O₃=1.5 to 4, for the purpose of manufacturing a steel with fewer Al clusters. Furthermore, Patent Document 2 discloses a technique that restricts an Al concentration in a steel to 0.001 to 0.002% and specifies a refractory material to be processed into a ladle and a slag composition for the purpose of manufacturing a saw wire steel.

Patent Document 3 discloses a technique that restricts an Al concentration in a steel to 0.005% or less for the purpose of exhibiting good durability against damage due to rolling-contact fatigue and ensuring excellent rolling-contact fatigue life. Patent Document 4 discloses a technique that restricts an Al concentration in a steel to 0.003% or less for the purpose of providing a high cleanliness Si-deoxidized steel that can achieve stable extension and refining of oxide-based inclusions in hot-rolling and cold-rolling processes.

Patent Document 5 discloses a technique that restricts an Al concentration to 0.003% or less, a Ti concentration to 0.0010% or less, a Zr concentration to 0.0001% or less, and a Rare-Earth Metal (REM) concentration to 0.0005% or less in a steel for the purpose of manufacturing a Si-deoxidized steel that can decrease the whole oxygen concentration in the Si-deoxidized steel to 15 ppm or less. Patent Document 6 discloses a technique that restricts an Al concentration to 0.003% or less, a Ti concentration to 0.003% or less, and a Zr concentration to 0.0010% or less in a steel for the purpose of providing a Si-deoxidized steel with excellent fatigue strength.

CONVENTIONAL ART DOCUMENT Patent Document

-   Patent Document 1: JP 2014-037598 A -   Patent Document 2: JP 2013-224480 A -   Patent Document 3: JP 2010-7092 A -   Patent Document 4: JP 2010-202905 A -   Patent Document 5: JP 2002-194497 A -   Patent Document 6: JP 2002-167647 A

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

Based on a close investigation of Patent Documents 1 to 6 mentioned above, there is no document that specifically describes the adjustment of a TiO₂ concentration in a slag, while some documents describe the techniques regarding the Ti concentration in the steel. For this reason, in practice, even the use of these techniques cannot precisely control a small Ti concentration contained in a steel. Thus, variations in the small Ti concentration in the steel are difficult to suppress.

Accordingly, the present invention has been made in view of the foregoing points, and it is an object of the present invention to provide a method for controlling a Ti concentration in a steel that can suppress variations in the Ti concentration in the steel.

Means for Solving the Problems

The method for controlling a Ti concentration in a steel according to the present invention is a method for controlling a Ti concentration in a steel when manufacturing a silicon-deoxidized steel including 0.1 to 3% by mass of Si and 0.0001 to 0.005% by mass of Al by ladle refining of a molten steel, the method comprising the step of:

adding an oxide, including TiO₂, to a slag in a ladle during the ladle refining, wherein the slag produced at end of the ladle refining satisfies formulas (1) to (7) below:

0.5≦CaO/SiO₂≦1.8  (1)

4% by mass≦Al₂O₃≦20% by mass  (2)

MgO≦15% by mass  (3)

1.5% by mass≦TiO₂≦10% by mass  (4)

CaO+SiO₂+Al₂O₃+MgO+TiO₂≧90% by mass  (5)

0.4≦TiO₂/MnO≦5  (6)

1≦TiO₂/T.Fe≦10  (7)

where a compound represented by a chemical formula represents the content of the compound in percent by mass; and T. Fe represents the total concentration, in mass ratio, of Fe contained in Fe oxides in the slag.

The method for manufacturing a silicon-deoxidized steel according to the present invention is a method for manufacturing a silicon-deoxidized steel including 0.1 to 3% by mass of Si and 0.0001 to 0.005% by mass of Al by ladle refining of a molten steel, wherein a Ti concentration in the steel is controlled by the method for controlling a Ti concentration in a steel according to the present invention.

Effects of the Invention

The present invention can suppress variations in the Ti concentration contained in the steel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows [Ti] concentrations in molten irons in Example A and Comparative Example D.

FIG. 2 shows [Ti] concentrations in molten irons in Example B and Comparative Example E.

FIG. 3 shows [Ti] concentrations in molten irons in Example C and Comparative Example F.

MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described below with reference to the accompanying drawings.

In a steelmaking process, after the end of refining a molten iron (molten steel) in a converter or an electric furnace, generally, the molten iron is poured into a ladle, and subjected to refining in the ladle (ladle refining). Oxygen used for decarburization or dephosphorization in the converter or electric furnace remains in the molten steel poured in the ladle. Thus, in the ladle refining or the like, for example, Al is added to the molten steel, thereby performing deoxidization to remove oxygen from the molten steel. In the deoxidization with Al, alumina or spinel generated in the molten steel by the addition of Al and the like is contained as non-metallic inclusions in the steel material in some cases. Taking this matter into consideration, there is a deoxidization method of a molten steel in which the amount of added Al is restricted as much as possible, and instead of that, Si which has less affinity for oxygen than Al is added to the molten steel. The present invention is directed to a process for deoxidizing a molten steel by adding at least Si to the molten steel.

In more detail, the present invention is directed to a method for manufacturing a silicon-deoxidized steel by ladle refining of a molten steel tapped from a converter or an electric furnace, the silicon-deoxidized steel including 0.1 to 3% (in percent by mass; the same shall apply hereinafter) of Si and 0.0001 to 0.005% of Al. The lower limit of Si concentration is 0.1%, while the upper limit of Si concentration is 3%. When the Si concentration exceeds 3%, that is, when the amount of Si added to the molten steel is large and the molten steel is strongly deoxidized, non-metallic inclusions containing silica as a main component would be included in the molten steel. Thus, the upper limit of Si concentration is set at 3%. Note that although the present invention is based on the assumption that after refining in the converter or electric furnace, ladle refining is carried out, this is not limited to the converter or electric furnace. Alternatively, the present invention may employ any type of refining furnace (steelmaking furnace) that can reduce a carbon concentration [C] in a molten steel to 2% or less before the ladle refining.

Regarding the ladle refining, the slag composition at the end of the ladle refining satisfies the following conditions: CaO/SiO₂ is 0.5 or more and 1.8 or less; Al₂O₃ is 4% or more and 20% or less; and MgO is 15% or less (excluding 0%). Note that a chemical formula, such as “CaO” and “SiO₂”, mentioned in the formula using at least one of “/”, “≦”, and “+” in the present specification means a content, in mass ratio, of a compound represented by the chemical formula.

The slag composition will be described below in more detail.

When CaO/SiO₂ is less than 0.5, non-metallic inclusions that contain SiO₂ as a main component tend to be generated, which degrades the fatigue properties of a steel material. Thus, the lower limit of CaO/SiO₂ is set at 0.5. In contrast, when CaO/SiO₂ exceeds 1.8, refractory material applied to the inner side of the ladle is susceptible to erosion by the slag. In other words, the amount of eroded refractory material increases, leading to an increase in the amount of non-metallic inclusions in the steel that is caused by the erosion of the refractory material. Thus, the upper limit of CaO/SiO₂ is set at 1.8.

When the Al₂O₃ content is less than 4%, the SiO₂ content is relatively more than the Al₂O₃ content, making it more likely to generate non-metallic inclusions containing SiO₂ as a main component. Thus, the lower limit of Al₂O₃ content is set at 4%. On the other hand, when the Al₂O₃ content is more than 20%, non-metallic inclusions containing Al₂O₃ as a main component are more likely to be generated, degrading the fatigue strength of the steel. Thus, the upper limit of Al₂O₃ content is set at 20%.

MgO is sometimes trapped inevitably into the steel from refractory material applied to the inner side of the ladle. Alternatively, MgO is sometimes added to reduce the melting point or viscosity of a slag. Here, when the MgO content exceeds 15%, non-metallic inclusions containing MgO as a main component are generated to degrade the fatigue strength of the steel. Thus, the upper limit of MgO content is set at 15%.

In the present invention, when manufacturing the silicon-deoxidized steel, an oxide including TiO₂ is added in the ladle refining process, whereby the TiO₂ content is set at 1.5 to 10%. That is, TiO₂ is essentially contained in the slag. TiO₂ is not contained as an impurity of a flux (plowing agent), but contained in the slag intentionally.

Next, a detailed description will be given on the addition of TiO₂ to the slag.

The inventors have tried to control a Ti concentration in a molten steel during ladle refining when manufacturing a silicon-deoxidized steel, i.e., a Si-killed steel by adding sponge Ti, pure Ti, or an alloy iron containing Ti metal, such as FeTi, to the molten steel. Besides, a ratio of distribution of Ti between the slag and the molten iron during the ladle refining, i.e., Ti distribution=(Ti)/[Ti] has been determined: where (Ti) is a Ti concentration (% by mass) in the slag, and [Ti] is a Ti concentration (% by mass) in the molten steel.

As a result, when adding sponge Ti, pure Ti, or an alloy iron containing Ti directly to the molten steel, the Ti distribution was a high value ranging from approximately 200 to 20000. This shows that even the addition of the pure Ti, such as sponge Ti, or the alloy iron containing Ti metal, such as FeTi, to the molten iron causes most of them to be present as a Ti oxide in the slag, wasting expensive titanium.

When the slag contains an oxide (FeO_(x), MnO, SiO₂, etc.) of an element having weaker oxidizing power, such as Fe, Mn, or Si, than Ti, the addition of Ti might reduce part of such an oxide. It is difficult to estimate the amount of Ti consumed for the reduction, consequently making it difficult to control the Ti concentration in the molten iron or the slag composition with high accuracy.

Accordingly, the inventors have found that TiO₂ should be added to a slag from an initial stage of the ladle refining, causing Ti to be distributed from the slag to the molten steel, so that the Ti concentration in the molten steel can be controlled with higher accuracy. Furthermore, this method has been found to be capable of suppressing the usage of expensive Ti metal or a Ti-containing alloy iron. In particular, when Ti in the molten iron is controlled to a minute concentration of several ppms to several tens of ppms, the method of adding TiO₂ to the slag is found to be performed with higher control accuracy than a method of introducing Ti metal to a molten steel.

To surely distribute the dissolved Ti in the molten steel, the TiO₂ concentration in the slag needs to be 1.5% or more, and preferably 2% or more. On the other hand, when the TiO₂ concentration in the slag exceeds 10%, non-metallic inclusions containing nitride TiN as a main component tend to be generated. Thus, the limit of TiO₂ concentration is set at 10%.

Furthermore, in the present invention, with regard to the slag composition at the end of the ladle refining, the relationship among CaO, SiO₂, Al₂O₃, MgO, and TiO₂ satisfies the following formula: CaO+SiO₂+Al₂O₃+MgO+TiO₂≧90%. Examples of the balance contained in the slag, other than CaO, SiO₂, Al₂O₃, MgO, and TiO₂, which are inevitably trapped into the slag, include MnO, FeO, Fe₃O₄, Fe₂O₃, P₂O, V₂O₅, etc. To enhance the accuracy of Ti distribution, the concentration of impurities needs to be less than 10%. That is, CaO+SiO₂+Al₂O₃+MgO+TiO₂≧90%.

Among elements inevitably trapped into the slag, MgO and Fe oxides (T.Fe), such as FeO, Fe₃O₄, and Fe₂O₃, are components that increase variations in the Ti distribution ratio. For this reason, in the present invention, the upper and lower limits of MgO and T.Fe have been specified.

It may be possible that the total of both the MnO concentration and the T.Fe concentration is specified when intending to specify these MnO and T.Fe concentrations. However, for the silicon-deoxidized steel, the MnO and the T.Fe need to be separately from each other. That is, for an Al-killed steel, Al has a strong power for reducing the slag, and thus a difference between the MnO concentration and the Fe oxide concentration in the slag is small. For a silicon-deoxidized steel, the power for reducing the slag is weak, compared to Al, which is more likely to make a difference between the MnO concentration and Fe oxide concentration. Additionally, when the Mn concentration is high, the difference between the MnO concentration and the T.Fe concentration in the slag becomes more larger. Thus, the silicon-deoxidized steel in the present invention separately specifies the MnO component and the T.Fe component in the slag.

In detail, with regard to MnO, TiO₂/MnO needs to be set at 0.4 or more and 5 or less. With regard to T.Fe, TiO₂/T.Fe needs to be set at 1 or more and 10 or less. In this way, the relationships of 0.4≦TiO₂/MnO≦5, and 1≦TiO₂/T.Fe≦10 are satisfied, thereby making it possible to suppress variations in the Ti distribution ratio. Note that T.Fe represents the total concentration, in mass ratio, of Fe contained in Fe oxides in the slag.

As mentioned above, in the present invention, when manufacturing the silicon-deoxidized steel, the slag composition at the end of the ladle refining satisfies the following conditions: CaO/SiO₂ is 0.5 or more and 1.8 or less; Al₂O₃ is 4% or more and 20% or less; and MgO is 15% or less (excluding 0%). Further, the addition of the oxide containing TiO₂ sets a TiO₂ content in the slag at 1.5 to 10%. The slag composition satisfies the following conditions: CaO+SiO₂+Al₂O₃+MgO+TiO₂≧90% by mass; 0.4≦TiO₂/MnO≦5; and 1≦TiO₂/T.Fe≦10.

The slag composition satisfies the above-mentioned conditions, so that the Ti concentration in the steel material can be set at 0.0005% to 0.01%. Note that with regard to the Ti concentration, the Ti concentration exceeding 0.01% can also be applied in principle, but in this case, a TiO₂ content in the slag needs to be more than 10%. In such a case, introducing Ti metal or an alloy containing Ti directly to a molten steel is sometimes more economical rather than the addition of TiO₂ to the slag. In addition, as mentioned above, when the TiO₂ content is set at more than 10%, non-metallic inclusions containing a nitride TiN as a main component tend to be generated. This is why the upper limit of a Ti concentration in the steel material is set at 0.01%.

On the other hand, Ti is trapped from impurities of an alloy iron containing Mn, Cr, Si, etc., other than Ti, a refractory material of a ladle, and a slag raw material other than TiO₂ in some cases. For this reason, the Ti concentration in the steel is sometimes in a range of approximately 0.0002 to 0.0004% without intentionally adding Ti or TiO₂. Thus, the lower limit of Ti concentration of the steel is 0.0005%.

Examples

Tables 1 and 2 collectively show Examples in which the control method of the Ti concentration in the steel according to the present invention was performed, as well as Comparative Examples in which a method different from that of the present invention was used for a process.

Note that “%” shown in the respective components of the “slag composition” in Tables 1 and 2 means o by mass.

TABLE 1 Total Amount amount of FeTi of Ti Amount of alloy Amount added to Slag composition electrolytic added to of TiO₂ molten Total of iron used molten added to iron and CaO MgO SiO₂ Al₂O₃ TiO₂ components Sample for melting iron slag slag (1) (2) (3) (4) (5) MnO T, Fe (1) to (5) No. (kg) (g) (g) (g) (%) (%) (%) (%) (%) (%) (%) (%) Example 1 2.0 0.00 0.80 0.48 31.9 6.1 45.8 5.5 1.8 2.9 1.4 91.1 A 2 2.0 0.00 0.80 0.48 32.5 3.8 46.0 6.5 2.0 3.1 1.0 90.8 3 2.0 0.00 0.80 0.48 33.0 4.1 45.1 5.9 2.2 2.8 1.2 90.3 4 2.0 0.00 0.80 0.48 35.1 3.5 44.2 6.4 1.9 3.0 0.9 91.1 5 2.0 0.00 0.80 0.48 36.3 2.9 43.9 6.0 2.5 3.8 0.8 91.6 6 2.0 0.00 0.80 0.48 37.1 3.2 43.1 5.4 3.0 4.1 1.0 91.8 7 2.0 0.10 0.70 0.49 35.1 4.1 44.0 6.1 3.1 3.3 1.2 92.4 Example 1 2.0 0.00 3.30 1.98 22.8 12.8 44.6 7.9 8.7 3.4 0.9 96.8 B 2 2.0 0.00 3.30 1.98 30.4 4.7 42.1 10.5 9.5 2.5 1.3 97.2 3 2.0 0.00 3.30 1.98 29.7 5.1 41.4 9.3 9.9 3.1 1.0 95.4 4 2.0 0.00 3.30 1.98 31.8 4.9 38.4 10.7 9.0 3.5 1.1 94.8 5 2.0 0.00 3.30 1.98 33.0 4.3 38.2 10.5 9.1 3.3 1.2 95.1 6 2.0 0.00 3.30 1.98 23.9 14.1 44.7 4.1 9.2 2.9 1.0 96.0 7 2.0 0.25 3.00 1.97 45.8 7.7 26.2 6.8 9.5 2.2 1.1 96.0 Example 1 2.0 0.00 2.30 1.38 40.1 3.1 42.9 5.2 4.4 3.5 1.1 95.7 C 2 2.0 0.00 2.30 1.38 39.9 4.7 38.2 7.6 5.2 1.6 0.7 95.6 3 2.0 0.00 2.30 1.38 44.0 5.7 29.9 6.5 5.0 2.3 1.0 91.1 4 2.0 0.00 2.30 1.38 40.5 4.4 37.1 4.9 4.2 1.9 0.9 91.1 5 2.0 0.00 2.30 1.38 38.5 5.1 40.4 4.0 4.8 4.1 0.9 92.8 6 2.0 0.00 2.30 1.38 38.7 5.2 38.2 7.6 5.1 2.2 0.9 94.8 7 2.0 0.00 2.30 1.38 39.9 7.9 34.1 10.0 5.0 2.5 1.5 96.9 Average Slag composition and Ti CaO/ TiO₂/ TiO₂/ Molten iron composition after Standard distribution Sample SiO2 MnO T. Fe adjustment (mass %) variation (Ti)/[Ti] No. (—) (—) (—) C Si Mn Cr Al Ti of Ti (—) Example 1 0.70 0.62 1.29 0.55 1.43 0.77 0.22 0.0005 0.0008 Average 1349 A 2 0.71 0.65 2.00 0.61 1.47 0.81 0.15 0.0006 0.0011 0.00107 1090 3 0.73 0.79 1.83 0.65 1.52 0.92 0.23 0.0004 0.0012 Standard 1099 4 0.79 0.63 2.11 0.63 1.97 0.87 0.17 0.0007 0.0020 deviation 570 5 0.83 0.66 3.13 0.67 2.20 0.86 0.19 0.0006 0.0011 0.00048 1362 6 0.86 0.73 3.00 0.65 2.00 0.87 0.19 0.0001 0.0008 2248 7 0.80 0.94 2.58 0.64 2.38 0.85 0.22 0.0002 0.0005 3717 Example 1 0.51 2.56 9.67 0.71 0.22 0.52 0.02 0.0001 0.0050 Average 1043 B 2 0.72 3.80 7.31 0.77 0.24 0.50 0.02 0.0003 0.0077 0.00754 740 3 0.72 3.19 9.90 0.80 0.25 0.51 0.02 0.0002 0.0095 Standard 625 4 0.83 2.57 8.18 0.81 0.20 0.47 0.02 0.0012 0.0054 deviation 999 5 0.86 2.76 7.58 0.82 0.21 0.50 0.01 0.0017 0.0072 0.00179 758 6 0.53 3.17 9.20 0.91 0.19 0.46 0.02 0.0001 0.0089 620 7 1.75 4.32 8.64 0.93 0.20 0.50 0.13 0.0043 0.0091 626 Example 1 0.93 1.26 4.00 0.99 0.20 0.34 1.50 0.0003 0.0025 Average 1055 C 2 1.04 3.25 7.43 1.00 0.24 0.33 1.46 0.0009 0.0031 0.00400 1006 3 1.47 2.17 5.00 0.98 0.24 0.34 1.51 0.0012 0.0057 Standard 526 4 1.09 2.21 4.67 1.00 0.24 0.34 1.49 0.0007 0.0050 deviation 504 5 0.95 1.17 5.33 0.99 0.22 0.31 1.41 0.0010 0.0032 0.00119 899 6 1.01 2.32 5.67 1.05 0.25 0.35 1.47 0.0008 0.0049 624 7 1.17 2.00 3.33 0.98 0.24 0.35 1.45 0.0009 0.0036 833

TABLE 2 Total Amount amount of FeTi of Ti Amount of alloy Amount added to Slag composition electrolytic added to of TiO₂ molten Total of iron used molten added to iron and CaO MgO SiO₂ Al₂O₃ TiO₂ components Sample for melting iron slag slag (1) (2) (3) (4) (5) MnO T, Fe (1) to (5) No. (kg) (g) (g) (g) (%) (%) (%) (%) (%) (%) (%) (%) Comparative 1 2.0 0.70 0.00 0.49 33.1 5.5 46.4 5.7 1.6 2.8 1.1 92.3 Example D 2 2.0 0.70 0.00 0.49 34.3 5.4 47.2 6.2 2.1 2.1 0.9 95.2 3 2.0 0.70 0.00 0.49 35.2 6.2 44.3 5.9 2.2 3.0 1.2 93.8 4 2.0 0.70 0.00 0.49 36.0 3.8 43.2 7.0 1.1 3.2 2.1 91.1 5 2.0 0.70 0.00 0.49 36.9 4.1 44.1 7.8 2.4 2.9 1.1 95.3 6 2.0 0.70 0.00 0.49 36.7 4.1 43.2 6.5 1.5 1.6 0.9 92.0 7 2.0 0.70 0.00 0.49 36.1 4.6 44.1 5.5 1.7 3.0 1.4 92.0 Comparative 1 2.0 2.80 0.00 1.96 22.8 12.8 44.6 7.9 6.0 1.1 0.8 94.1 Example E 2 2.0 2.80 0.00 1.96 30.4 4.7 42.1 10.5 7.1 1.4 0.9 94.8 3 2.0 2.80 0.00 1.96 29.7 5.1 41.9 9.3 8.1 1.4 1.2 94.1 4 2.0 2.80 0.00 1.96 31.8 4.9 38.4 10.7 9.1 1.2 0.4 94.9 5 2.0 2.80 0.00 1.96 33.0 4.3 38.2 10.5 9.0 1.5 0.3 95.0 6 2.0 2.80 0.00 1.96 23.9 14.1 44.7 4.1 7.4 4.4 1.5 94.2 7 2.0 2.80 0.00 1.96 45.8 7.7 26.2 6.8 8.2 3.1 1.4 94.7 Comparative 1 2.0 2.00 0.00 1.40 40.1 3.1 42.9 5.2 6.4 1.1 0.4 97.7 Example F 2 2.0 2.00 0.00 1.40 39.9 4.7 38.2 7.6 7.0 0.9 0.4 97.4 3 2.0 2.00 0.00 1.40 44.0 5.7 29.9 6.5 6.0 2.3 1.0 92.1 4 2.0 2.00 0.00 1.40 40.5 4.4 37.1 4.9 6.1 1.9 0.9 93.0 5 2.0 2.00 0.00 1.40 38.5 5.1 40.4 4.0 6.7 3.1 0.9 94.7 6 2.0 2.00 0.00 1.40 38.7 5.2 38.2 7.6 5.1 2.1 1.0 94.8 7 2.0 2.00 0.00 1.40 39.9 7.9 34.1 10.0 5.0 2.5 1.5 96.9 Average Slag composition and Ti CaO/ TiO₂/ TiO₂/ Molten iron composition after Standard distribution Sample SiO2 MnO T. Fe adjustment (mass %) variation (Ti)/[Ti] No. (—) (—) (—) C Si Mn Cr Al Ti of Ti (—) Comparative 1 0.71 0.57 1.45 0.57 1.42 0.75 0.17 0.0005 0.0002 Average 4796 Example D 2 0.73 1.00 2.33 0.60 1.44 0.80 0.16 0.0006 0.0003 0.00113 4196 3 0.79 0.73 1.83 0.64 1.50 0.91 0.21 0.0004 0.0003 Standard 4396 4 0.83 0.34 0.52 0.63 1.97 0.92 0.22 0.0007 0.0042 deviation 157 5 0.84 0.83 2.18 0.68 2.22 0.85 0.19 0.0006 0.0003 0.00144 4796 6 0.85 0.94 1.67 0.64 2.31 0.84 0.18 0.0001 0.0011 817 7 0.82 0.57 1.21 0.64 2.04 0.84 0.21 0.0002 0.0015 679 Comparative 1 0.51 5.45 7.50 0.73 0.24 0.62 0.01 0.0001 0.0069 Average 521 Example E 2 0.72 5.07 7.89 0.77 0.23 0.59 0.02 0.0001 0.0083 0.00690 513 3 0.71 5.79 6.75 0.81 0.24 0.57 0.01 0.0002 0.0121 Standard 401 4 0.83 7.58 22.75 0.81 0.25 0.51 0.02 0.0002 0.0010 deviation 5455 5 0.86 6.00 30.00 0.84 0.20 0.49 0.01 0.0016 0.0007 0.00443 7708 6 0.53 1.68 4.93 0.92 0.20 0.46 0.10 0.0020 0.0092 482 7 1.75 2.65 5.86 0.92 0.20 0.49 0.13 0.0040 0.0101 487 Comparative 1 0.93 5.82 16.00 1.01 0.19 0.33 1.51 0.0005 0.0002 0.00433 19184 Example F 2 1.04 7.78 17.50 1.03 0.25 0.33 1.49 0.0004 0.0002 Standard 20982 3 1.47 2.61 6.00 0.99 0.25 0.35 1.49 0.0005 0.0076 deviation 473 4 1.09 3.21 6.78 1.00 0.24 0.37 1.50 0.0011 0.0061 0.00382 599 5 0.95 2.16 7.44 1.04 0.18 0.31 1.48 0.0020 0.0021 1913 6 1.01 2.43 5.10 1.02 0.19 0.32 1.46 0.0015 0.0102 300 7 1.17 2.00 3.33 0.99 0.19 0.32 1.50 0.0005 0.0039 769

Implementation conditions for Examples and Comparative Examples will be described below.

In each of Examples and Comparative Examples, 2 kg of electrolytic iron was melted in an alumina crucible by using an induction melting furnace. Note that the electrolytic iron in use was a commercially available one. Furthermore, its purity does not matter. Although material for the crucible in use was alumina (Al₂O₃), any material that can withstand the erosion of the molten steel (molten iron) or slag at a temperature of 1,550° C. or higher may be used for the crucible. For example, magnesia (MgO) may be used. Note that to prevent oxidization of the molten iron, the electrolytic iron was melted under an Ar airflow.

The molten steel temperature was kept in a range of 1,550 to 1,570° C. by adjusting an output of the induction melting furnace after melting raw materials. In adjustment of the components, predetermined amounts of carbon particles, silicon wafer scraps, electrolytic manganese, and an Fe—Cr—C alloy were added to the molten iron, thereby adjusting [C], [Si], [Mn], and [Cr] in the molten iron at predetermined concentrations. In Comparative Example, an Fe—Ti alloy in which a Ti content was 70% was added to the molten iron to adjust the Ti concentration in the molten iron. As long as a concentration of a target element is previously known, any kind of additive for adjusting the components can be used. For example, an Fe—Si alloy, an Fe—Mn alloy, an Fe—Si—Mn alloy, or the like may be combined.

The components of the molten iron were as follows: C=0.5 to 1.1%; Si=0.14 to 2.3%; Mn=0.16 to 0.92%; Cr=0 to 1.6%; and Al=0.0001 to 0.005%.

After adjusting the components of the molten iron, 40 g of a flux having a predetermined composition was added to the molten iron.

To prepare the flux with the predetermined composition, a commercially available oxide reagent or carbonate reagent (SiO₂, CaCO₃, MgO, Al₂O₃, MnO₂, TiO₂) was previously mixed together, which was burned at 1,500° C. for 8 hours.

After adding the flux, the molten iron with the flux were held at a predetermined temperature for 60 minutes, followed by turning off the induction melting furnace to thereby solidify the molten iron and the slag in the crucible. Thereafter, predetermined amounts of samples were cut out of the molten irons and slags, and their respective components of the samples were analyzed.

Regarding Examples and Comparative Examples, Examples A, B, and C are examples in which TiO₂ was added as the flux, while Comparative Examples D, E, and F are examples in which TiO₂ was not added at all. Example A-Sample 7 and Example B-Sample 7 were examples in which both TiO₂ and Ti alloy were added.

Example A and Comparative Example D handled a group of silicon strongly-deoxidized steels in which the target [Ti] concentration in the molten iron was 0.001%. In each of Example A and Comparative Example D, the total pure Ti content added as FeTi or TiO₂ was 0.48 to 0.49 g.

Example B and Comparative Example E handled a group of silicon weakly-deoxidized steels in which the target [Ti] concentration in the molten iron was 0.007%. In each of Example B and Comparative Example E, the total pure Ti content added as FeTi or TiO₂ was 1.96 to 1.98 g. Furthermore, Example C and Comparative Example F handled a group of Cr-added silicon weakly-deoxidized steels in which a target [Ti] concentration in a molten iron was 0.004%.

In Example C and Comparative Example F, the total pure Ti content added as FeTi or TiO₂ was 1.38 to 1.40 g. Note that the silicon strongly-deoxidized steels, silicon weakly-deoxidized steels, and Cr-added silicon weakly-deoxidized steels are silicon-deoxidized steels. The definitions of the terms “Si strongly-deoxidized steel” and “Si weakly-deoxidized steel” are not distinguished by their [Si] concentrations, but these terms are used for relative comparison between the steel groups convenience.

Examples and Comparative Examples were compared and evaluated in terms of variations in the after adjusted [Ti] concentration in the molten iron of the same kind of steel group mentioned above, i.e., by the standard deviation.

In each of Examples (Example A, Example B, and Example C), the slag composition satisfied the following conditions: CaO/SiO₂=5 to 1.8; Al₂O₃=4 to 20%; and MgO≦15% (excluding 0%), and the oxide containing TiO₂ was added, resulting in TiO₂=1.5 to 10%. Furthermore, the slag satisfied the following conditions: CaO+SiO₂+Al₂O₃+MgO+TiO₂≧90%; 0.4≦TiO₂/MnO≦5; and 1≦TiO₂/T.Fe≦10.

On the other hand, in any of Comparative Examples (Comparative Example D, Comparative Example E, and Comparative Example F), no oxide containing TiO₂ was added (i.e., the amount of TiO₂ added to the slag was zero).

FIGS. 1 to 3 are diagrams collectively showing Examples and Comparative Examples.

FIG. 1 shows [Ti] concentrations of molten irons in Example A and Comparative Example D; FIG. 2 shows [Ti] concentrations of molten irons in Example B and Comparative Example E; and FIG. 3 shows [Ti] concentrations of molten irons in Example C and Comparative Example F.

As shown in FIG. 1, when comparing Example A with Comparative Example D, the average [Ti] concentrations in the molten irons were substantially the same. However, in Comparative Example D, the standard deviation was 0.00144, while in Example A, the standard deviation could be a small value, i.e., 0.00048. As shown in FIG. 2, when comparing Example B with Comparative Example E, the average [Ti] concentrations in the molten irons were substantially the same. However, in Comparative Example D, the standard deviation was 0.00443, while in Example B, the standard deviation could be a small value, i.e. 0.00179. As shown in FIG. 3, when comparing Example C with Comparative Example F, the average [Ti] concentrations in the molten irons were substantially the same. However, in Comparative Example F, the standard deviation was 0.00382, while in Example C, the standard deviation could be a small value, i.e., 0.00119.

In other words, by the method for controlling the Ti concentration in the steel according to the present invention, the steels had a Ti concentration in a range of 0.0005% to 0.01%. Such steels in Examples could reduce variations in the Ti concentration to approximately ⅓ to ⅖ of those in Comparative Examples.

In the embodiments disclosed herein, matters not clearly mentioned, including operating conditions, various parameters, and the dimension, weight, and volume of components, adopt the matters that could be easily conceived by a person skilled in the art without departing from the scope of implementation by the person skilled in the art.

The present application claims priority to Japanese Patent Application No. 2014-168472 filed on Aug. 21, 2014, the disclosure of which is incorporated herein by reference in its entirety. 

1. A method for controlling a Ti concentration in a steel when manufacturing a silicon-deoxidized steel comprising 0.1 to 3% by mass of Si and 0.0001 to 0.005% by mass of Al by ladle refining of a molten steel, the method comprising the step of: adding an oxide including TiO₂ to a slag in a ladle during the ladle refining, wherein the slag produced at end of the ladle refining satisfies formulas (1) to (7) below: 0.5≦CaO/SiO₂≦1.8  (1) 4% by mass≦Al₂O₃≦20% by mass  (2) MgO≦15% by mass  (3) 1.5% by mass≦TiO₂≦10% by mass  (4) CaO+SiO₂+Al₂O₃+MgO+TiO₂≧90% by mass  (5) 0.4≦TiO₂/MnO≦5  (6) 1≦TiO₂/T.Fe≦10  (7) where a compound represented by a chemical formula represents the content of the compound in percent by mass; and T.Fe represents the total concentration, in mass ratio, of Fe contained in Fe oxides in the slag.
 2. A method for manufacturing a silicon-deoxidized steel comprising 0.1 to 3% by mass of Si and 0.0001 to 0.005% by mass of Al by ladle refining of a molten steel, wherein a Ti concentration in the steel is controlled by the method for controlling a Ti concentration in a steel according to claim
 1. 